1. Field of the Invention
The present invention relates to the field of photonic circuitry, and more specifically to a method of manufacturing an integrated optical device.
2. Background of the Related Art
Integrated optical devices comprising optical components in discrete areas of a substrate are known. An example is an arrayed waveguide grating (hereafter referred to as an AWG) as commonly used for wavelength division multiplexing (WDM) in the field of optical telecommunications. A typical application of an AWG is to split light of multiple wavelengths from a single incoming optical fiber and send each wavelength to a different output fiber. In this way, signals may be transmitted on different optical wavelengths in the same fiber and then separated at the destination.
In practice, an AWG is usually integrated on a substrate by providing a series of input waveguides and a series of output waveguides, with an array of curved waveguides there between and separated from the input and output waveguides by a coupling (or xe2x80x9cslabxe2x80x9d) region (see FIG. 1 described further below). Such an AWG is described further in U.S. Patent No. 5,002,350, the disclosure of which is incorporated by reference herein.
AWGs enable the spatial separation of optical wavelengths; this separation is a result of the optical path differences along each of the curved waveguides in the array. Typically, the lengths of the waveguides in the array are carefully designed so that each one is a constant distance longer than its shorter neighbor. Regardless of whether the increment is constant or varying, all AWGs require the lengths of the waveguides to be accurately calculated, defined and manufactured. If such devices are fabricated poorly, the errors in each of the waveguide path lengths causes a decrease in the isolation between neighboring wavelengths afforded by the device.
During the manufacture of AWGs it is necessary to transfer the design of the device onto a substrate (typically a silicon wafer) from which it is fabricated. Previously, this has been done using one of the following two methods:
a) using a 1:1 quartz/chromium optical mask and standard ultra-violet photoresist photolithography; or
b) directly writing the pattern into a thin layer of resist on the material""s surface by using a focused electron-beam.
Photolithographic stepper machines are known which contain imaging optics to reduce magnified design patterns, or masks, down to a required size on a material surface for example, in the fabrication of an integrated electronic circuit. Although such reduction provides a very accurate design image on the material surface, the finite size of the imaging optics imposes a restraint upon the maximum size of area (or xe2x80x9cfieldxe2x80x9d) which the stepper machine can define in a single exposure. Thus, typically, such machines reduce a series of magnified masks to produce a larger design by way of xe2x80x9csteppingxe2x80x9d between different fields. The adjacent fields are joined or aligned, known as xe2x80x9cstitchingxe2x80x9d, to produce the overall, larger design and although stitching accuracy (often less than 100 nm in either direction) is sufficient, to maintain electrical contact between parts of an integrated electrical circuit, a potential misalignment of this magnitude may have substantial adverse effects in an integrated optical circuit as it may, for instance, impair or even destroy the optical communication between two sections of a waveguide or substantially affect the operation of a device by-changing the optical path lengths of the device. Thus, there is a need in the art for improved processing techniques to define optical features in integrated optical devices.
The present invention provides a method of manufacturing integrated optical devices such as AWGs which enable the devices to be made more accurately and with increased production yields.
Thus, according to a first aspect of the present invention there is a method of manufacturing an integrated optical device on a substrate, the device comprising first and second areas each containing at least one optical component to be defined with the first and second areas being separated by a third, relatively featureless area which provides optical communication between the first and second areas. The method comprises the use of a photolithographic stepper to define features of the optical components on the substrate, and being arranged to define the features of the optical component(s) in the first area within a first exposure field and the features of the optical component(s) in the second area within a second exposure field. The stepper is moved between exposures so that the first and second exposure fields are contiguous, the arrangement being such that a line of contact or area of overlap between the first and second fields lies within the said third, relatively featureless area of the substrate.
By arranging that the line of contact or area of overlap between the first and second fields lies within the third relatively featureless area of the substrate, it is possible to avoid offset or misalignment in the optical components that would occur if the line of contact or area of overlap lay across the optical component. Such offset, for example, between two parts of a waveguide, would interfere with the operation of the optical device and may, for instance, prevent the transmission of an optical signal through the device or disturb the lengths of optical paths within the device. However, any offset within the third, relatively featureless area is of much less consequence, since the third area contains no optical components the operation of which can be disrupted by a small misalignment between the fields.
In yet a further aspect, the present invention provides an integrated optical component obtainable by the method described above.